Available methods for increasing the recording capacity of an optical disc are: increasing density of an optical disc and using multi-recording layers.
For example, in a Blu-ray disc, the recording capacity of one optical disc is increased to 50 GB by increasing the numerical aperture (NA) of an objective lens from 0.6 to 0.85 and decreasing the wavelength of the laser from 650 nm to 405 nm, so that density is increased, and by making the recording layer two layers.
Recently in order to increase capacity even more, development of a multilayer optical disc having a higher number of recording layers is in-progress.
When the NA is high and the wavelength is short, it is known that the recording and reproducing performance greatly deteriorates due to spherical aberration, which is generated depending on the thickness from the surface of the optical disc to each recording layer, where correction of the spherical aberration is indispensable.
In order to implement good recording and reproducing in each recording layer, it is necessary to correct spherical aberration to an optimum state in each recording layer.
During this spherical aberration correction operation, an operation of changing the focal point position of the light beam, from the current recording layer to another recording layer, may be performed. This operation of changing the focal point position of the light beam from the current layer to another recording layer is generally called a “focus jump”.
Spherical aberration not only deteriorates the recording and reproducing performance, but also deteriorates the quality of servo signals, which are used for focus control and tracking control.
In order to perform a stable focus jump operation in a multilayer optical disc having a plurality of recording layers, some methods related to the procedure for spherical aberration correction and focus jump were proposed.
A technology disclosed in Patent Literature 1, for example, is that spherical aberration is corrected prior to focus jump, so that an optimum state is generated at a mid-position between the current recording layer and a target recording layer at a shift destination, and spherical aberration is corrected again after the focus jump is executed so that an optimum state is generated in the target recording layer at the shift destination. According to Patent Literature 1, a stable focus jump operation can be implemented by this procedure.
A technology disclosed in Patent Literature 2 is that spherical aberration is corrected prior to focus jump, so that an optimum state is generated in a target recording layer at a shift destination, then the focus jump is executed. According to Patent Literature 2, stable focus jump operation can be implemented by this procedure.
A multilayer optical disc structure, as shown in Patent Literature 3, was proposed as a means of further increasing the capacity of optical discs using these technologies.
However with conventionally proposed methods, it is difficult to implement a stable interlayer shift if the number of layers of an optical disc is further increased.
In the case of a multilayer optical disc, the following restrictions are imposed on the layer structure.
(1) In order to ensure the protection of the recording and reproducing performance against dust and fingerprints which adhere to the surface where a light beam enters, at least 50 μm or more of thickness is required between the surface and the recording layer closest to the surface.
(2) Since the tilt margin must be ensured, the thickness between the surface and the recording layer that is most distant from the surface cannot be increased very much. For example, in the case of a Blu-ray disc, the thickness between the surface and the recording layer that is most distant from the surface is 100 μm.
(3) The interlayer distance between each recording layer cannot be the same. (If the interlayer distance among recording layers is the same, the reflected light from the focused recording layer and the reflected light from the rear face of the recording layer, which is two layers away from the focused recording layer, interfere.)
(4) The interlayer distance between each recording layer cannot be too short. (The shortest interlayer distance is about 10 μm, otherwise the interference from the adjacent recording layers increases.)
Therefore if it is attempted to implement an optical disc having four recording layers, the interlayer distance among recording layers becomes uneven, as shown in the layer structure in FIG. 11. FIG. 11 is a diagram depicting the layer structure of a multilayer optical disc.
The optical disc 31 shown in FIG. 11 comprises: a first recording layer L0 which is most distant from a surface 201 where a light beam enters, a second recording layer L1 which is the second most distant from the surface 201, a third recording layer L2 which is the third most distant from the surface 201, a fourth recording layer L3 which is closest to the surface 201, a cover layer 203 which is disposed between the fourth recording layer L3 and the surface 201, a first intermediate layer 204 which is disposed between the third recording layer L2 and the fourth recording layer L3, a second intermediate layer 205 which is disposed between the second recording layer L1 and the third recording layer L2, a third intermediate layer 206 which is disposed between the first recording layer L0 and the second recording layer L1, and a fourth intermediate layer 207 which is disposed between the first recording layer L0 and a label layer 202.
A recording layer which is most distant from the side which the light beam enters is called the first recording layer L0, and the other recording layers are called, in order from the first recording layer L0 to the surface of the optical disc, the second recording layer L1, the third recording layer L2 and the fourth recording layer L3. For example, the interlayer distance between the first recording layer L0 and the second recording layer L1 is 17 μm, the interlayer distance between the second recording layer L1 and the third recording layer L2 is 20 μm, the interlayer distance between the third recording layer L2 and the fourth recording layer L3 is 13 μm, and the interlayer distance between the fourth recording layer L3 and the surface 201 is 50 μm.
For example, Patent Literature 3 proposes a layer structure of a multilayer optical disc having four recording layers. FIG. 12A to FIG. 12D show the focus error signals obtained when the light beam enters this multilayer optical disc.
FIG. 12A to FIG. 12D are graphs showing the focus error signals which are detected when the spherical aberration is optimized for each recording layer of a multilayer optical disc. FIG. 12A is a graph showing a focus error signal acquired in a state where the spherical aberration is optimized for the first recording layer L0, FIG. 12B is a graph showing a focus error signal acquired in a state where the spherical aberration is optimized for the second recording layer L1, FIG. 12C is a graph showing a focus error signal acquired in a state where the spherical aberration is optimized for the third recording layer L2, and FIG. 12D is a graph showing a focus error signal acquired in a state where the spherical aberration is optimized for the fourth recording layer L3.
In FIG. 12A to FIG. 12D, the focus error signal 301 is a focus error signal corresponding to the first recording layer L0, the focus error signal 302 is a focus error signal corresponding to the second recording layer L1, the focus error signal 303 is a focus error signal corresponding to the third recording layer L2, and the focus error signal 304 is a focus error signal corresponding to the fourth recording layer L3.
Throughout all of FIG. 12A to FIG. 12D, a focus error signal in a recording layer, for which spherical aberration is not optimized, greatly deteriorates, so stable focus control cannot be implemented in this state.
Now a conventionally proposed method for interlayer shift from the third recording layer L2 to the second recording layer L1 is described. Before the interlayer shift, the focus error signal 303, corresponding to the third recording layer L2, can be sufficiently acquired, as shown in FIG. 12C, so stable focus control can be implemented. When the interlayer shift is performed, the spherical aberration is adjusted to an optimum state for the second recording layer L1, which is the target recording layer at the shift destination. In this state, the focus error signal 302, corresponding to the second recording layer L1, can be sufficiently acquired as shown in FIG. 12B.
At this point however, focus is still controlled to be on the third recording layer L2. Hence as shown in FIG. 12B, the symmetry of the focus error signal 303, corresponding to the third recording layer L2, is greatly affected. The reasons are as follows.
First the interlayer distance between the second recording layer L1 and the third recording layer L2 is long, so if the spherical aberration is adjusted to an optimum level for the second recording layer L1, the focus error signal of the third recording layer L2 deteriorates.
Second the interlayer distance between the third recording layer L2 and the fourth recording layer L3 is short, so the respective focus error signals adjacent to each other interfere, and amplitude deteriorates.
In such a state, it is difficult to implement stable focus control, and if offset voltage is generated in the focus error signals or if such disturbance as impact occurs during the operation, the focus jump operation becomes unstable, and focus control may be lost.
A configuration of a conventional optical disc apparatus will be described with reference to FIG. 13. FIG. 13 is a diagram depicting a configuration of a conventional optical disc apparatus.
The optical disc apparatus in FIG. 13 comprises an optical pickup 11, a focus actuator drive circuit 21, a tracking actuator drive circuit 22, a spherical aberration correction actuator drive circuit 23, a focus error signal generator 25, a tracking error signal generator 26, an RF signal generator 27, a reproducing signal quality index generator 28, a disc motor 29, a microcomputer 51 and a control unit 52.
The optical pickup 11 irradiates a light beam onto an optical disc 31, and reads information recorded on the optical disc 31. Or the optical pickup 11 irradiates a light beam onto an optical disc 31, and records information on the optical disc 31. The focus actuator drive circuit 21 displaces an objective lens 1 of the optical pickup 11 to be approximately vertical to the optical disc 31.
The optical disc apparatus shown in FIG. 13 records information on the optical disc 31 or reproduces information recorded on the optical disc 31. First light beams emitted from a laser light source 9, disposed on the optical pickup 11, become parallel beams by a collimator lens 8. These parallel beams pass through a spherical aberration correction unit 7, polarization beam splitter 10 and a ¼ wavelength plate 6, and converge on an information recording surface (recording film) of the optical disc 31 by the objective lens 1.
The reflected light from the optical disc 31 transmits through the objective lens 1 and the ¼ wavelength plate 6, and is then reflected by the polarization beam splitter 10 and reaches a light receiving unit 5. Here the optical disc 31 is rotary-driven by the disc motor 29.
The light receiving unit 5 converts the reflected light from the optical disc 31 into an electric signal. The output of the light receiving unit 5 is supplied to the focus error signal generator 25, tracking error signal generator 26 and RF signal generator 27.
The focus error signal generator 25 detects a positional shift between the focus position of the light beam irradiated onto the optical disc 31 and the information recording surface of the optical disc 31 based on the output of the light receiving unit 5, and outputs the detected positional shift as a focus error signal. The focus error signal can be generated by an astigmatism method, for example.
The tracking error signal generator 26 detects a positional shift between a spot of the light beam formed on the information recording surface of the optical disc 31 and a track on the information recording surface of the optical disc 31 based on the output of the light receiving unit 5, and outputs the detected positional shift as a tracking error signal. The tracking error signal can normally be generated by a push-pull method, for example.
The focus error signal and the tracking error signal are supplied to the control unit 52. The control unit 52 performs such signal processings as phase compensation on the focus error signal and tracking error signal, so as to generate control signals.
The focus actuator drive circuit 21 drives the focus actuator 2 disposed on the optical pickup 11 by supplying a drive signal to the focus actuator 2 according to a control signal from the control unit 52. The tracking actuator drive circuit 22 drives the tracking actuator 3 disposed on the optical pickup 11 by supplying a drive signal to the tracking actuator 3 according to a control signal from the control unit 52.
The focus actuator 2 drives an objective lens 1 according to a drive signal from the focus actuator drive circuit 21. The tracking actuator 3 also drives the objective lens 1 according to a drive signal from the tracking actuator drive circuit 22.
In this way, the control unit 52 forms a servo loop for focus control by controlling the focus actuator drive circuit 21 for driving the focus actuator 2, according to the focus error signal. Furthermore, the control unit 52 forms a servo loop for tracking control by controlling the tracking actuator drive circuit 22 for driving the tracking actuator 3 according to the tracking error signal. In this way servo control is executed.
The RF signal generator 27 generates an RF signal based on the output of the light receiving unit 5, and outputs the RF signal to the reproducing signal quality index generator 28. The reproducing signal quality index generator 28 generates a reproducing signal quality index which indicates the reproducing performance of the reproducing signal based on the RF signal acquired from the RF signal generator 27. The reproducing signal quality index is, for example, a jitter or an error rate. The reproducing signal quality index generated by the reproducing signal quality index generator 28 is supplied to the microcomputer 51.
The spherical aberration correction actuator drive circuit 23 corrects spherical aberration by supplying a drive signal to the spherical aberration correction unit 7 according to a control signal from the microcomputer 51. The correction amount of the spherical aberration can be determined according to the distance from the surface of the optical disc to each recording layer which is specified by the optical disc standard, for example.
The microcomputer 51 supplies a drive command value for performing the focus jump operation to the control unit 52. A drive signal based on this drive command value is supplied to the focus actuator drive circuit 21, and the focus actuator 2 starts driving.
Now the procedure of the focus jump operation from the third recording layer L2 to the second recording layer L1 will be described with reference to FIG. 14. FIG. 14 is a graph showing changes of the focus error signal 401, focus actuator drive output 402 and the signal 403 to indicate the spherical aberration correction amount in the case of a conventional interlayer shift. A state L420 shows a state where the spherical aberration corresponding to the first recording layer L0 is the optimum, a state L421 is a state where the spherical aberration corresponding to the second recording layer L1 is the optimum, the state L422 shows a state where the spherical aberration corresponding to the third recording layer L2 is the optimum, and the state L423 shows a state where the spherical aberration corresponding to the fourth recording layer L3 is the optimum. The initial state of a signal 403, to indicate the spherical aberration correction amount, is the state L422.
First at the timing T101, the spherical aberration correction amount is in state L422 when the recording or reproducing operation is being executed on the third recording layer L2. Then at the timing T102, the optical disc apparatus starts the spherical aberration correction operation. From the timing T102 to the timing T104, the optical disc apparatus adjusts the spherical aberration correction amount from the state L422 to the state L421, while continuing the focus control operation. However at the timing T103, while changing the spherical aberration correction amount from the state L422 to the state L421, focus control is lost. As a result, at the timing T105, the spot position of the light beam passes the surface of the optical disc, and the focus jump operation fails.
This is because, as described in FIG. 12B, when the spherical aberration correction amount is the optimum for the second recording layer L1, the focus error signal obtained from the third recording layer L2 becomes extremely asymmetric, and the stability of focus control versus axial run-out or disturbance of the optical disc drops.
As a result, the recording and reproducing operations for a desired recording layer may become impossible in a multilayer optical disc, so a stable focus jump method is demanded.
Furthermore, in the case of a multilayer optical disc, an increase in the number of recording layers widens the distance from a recording layer closest to the surface to a recording layer most distant from the surface, so spherical aberration must be corrected in a wide range corresponding to this interlayer distance. A generally used method to implement spherical aberration correction in such a wide range is moving the spherical aberration correction element by a stepping motor. However the size of the optical pickup is limited, and it is difficult to enclose a high output stepping motor, which size is large, in the optical pickup. Therefore a stepping motor for driving the spherical aberration correction element must be small and have low output, and in this case, it is difficult to increase the driving speed of the spherical aberration correction element. Additionally, drive control by the stepping motor is normally performed with a trapezoidal speed profile, in which acceleration or deceleration is performed gradually, in order to prevent a loss of synchronization, so the driving time becomes long if driving and stopping are performed continuously.
In the conventionally proposed focus jump procedure, a continuous interlayer shift among a plurality of recording layers in a multilayer optical disc is not considered. Hence in a four-layer disc, for example, the conventional focus jump procedure must be repeated a maximum of three times. But as mentioned above, it is difficult to increase the driving speed of the stepping motor which drives the spherical aberration correction element. Therefore the spherical aberration correction operation takes time, and the interlayer access time in a multilayer optical disc increases. If the interlayer access time increases, video images may be interrupted during the reproducing of a video image, which is not desirable. As a consequence, increasing the speed of interlayer access is demanded for multilayer optical discs.